Fault Tree Analysis concept

Fussell, J.B. Fault Tree Analysis Concepts and Techniques", Generic Techniques in System Reliability Assessment, 1976... 

Tliis cliapter is concerned willi special probability distributions and tecliniques used in calculations of reliability and risk. Tlieorems and basic concepts of probability presented in Cliapter 19 are applied to llie determination of llie reliability of complex systems in terms of tlie reliabilities of their components. Tlie relationship between reliability and failure rate is explored in detail. Special probability distributions for failure time are discussed. Tlie chapter concludes with a consideration of fault tree analysis and event tree analysis, two special teclmiques lliat figure prominently in hazard analysis and llie evaluation of risk. 

The overall concept of all of the following tools is that of risk analysis or risk assessment. Risk analysis helps to decide whether an aspect is GMP-critical or not. The risk analysis can be performed in a formal or more informal way. Following are two popular and import types of risk analysis. Another method, the fault tree analysis (FTA), has recently been used in the area of computer validation. This method is not described here, as it is a complex form of risk analysis. 

Toward the end of the Second World War, systems techniques such as fault tree analysis were introduced in order to predict the reliability and performance of military airplanes and missiles. The use of such techniques led to the formalization of the concept of probabilistic risk assessment (PRA). The publication of the Reactor Safety Study (NRC, 1975)—often referred to as the Rasmussen Report after the name of principal author, or by its subtitle WASH 1400—demonstrated the use of such techniques in the fledgling nuclear power business. Although WASH 1400 has since been supplanted by more advanced analysis techniques, the report was groundbreaking in its approach to system safety. 

Fault tree analysis is another commonly used system safety method. H. A. Watson at Bell Telephone Laboratories originated fault tree analysis in 1962. It is most suitable for complex systems. It is a Boolean logic concept that evaluates events. 

NFPA has developed a Fire Safety Concepts Tree At the top of the tree are fire safety objectives, followed by actions to achieve the objectives. Elements of the tree connect using AND and OR gates, similar to fault tree analysis (Figure 36-10). A Fire Safety Concepts Tree can help analyze buildings and designs using qualitative and quantitative procedures. 

System behavior analysis and prognosis can be executed by means of various procedures. The procedures generally described in the literature can be traced back to three standard types, namely failure effect analysis [4-9], fault tree analysis [4-10], and incident progression analysis [4-11], The three procedures will be discussed, as well as the concept of the decision table technique, which is also a good tool but has rarely been discussed in the literature in connection with this application. To begin, the customary analysis techniques [4-9], [4-10], [4-11] will be discussed in alphabetical order. This will serve to delineate and distinguish the procedures. 

Fault Tree Analysis for E/E/PE Concepts and Application Notes... 

Clements, P. L. 1987. Concepts in Risk Management. Sverdrup Technology, Inc. Crosetti, P. A. 1982. Reliability and Fault Tree Analysis Guide. DOE 76-45/22 SSDC-22. Idaho Falls, ID Department of Energy. 

Haasl, D. F 1965. Advanced concepts in fault tree analysis. System Safety Symposium, Seattle, WA. 

An important concept of fault tree analysis is that all the events and subevents are independent of each other. So for example in the CIRIA 152 fault tree the cause of ventilation cannot be dependent on a factor that also causes ignition (i.e. entry into the cupboard). 

The term engineering RA is mainly introduced to highlight differences to the IT RA concept. The framework is well defined in the (outdated) (ISO/ IECGuide73 2002). The basic approaches and concepts as, e.g., FMEA, Fault Tree Analysis and Probabilistic Safety Analysis, are supposed to be known to the reader. Major goals are hazard identification and its impact on environment. Typical fields of application are chemical industry and nuclear power generation. 

Functional specification and functional realization are the functional views of a component. These views are models that describe the desired data flow through a component on different levels of abstraction. Other functional and non-functional properties of a component, such as resource consumption, quality of services, or dependability, are modeled and separated by additional views (models). For example, the propagation of failures through a component is modeled by a failure specification and a failure realization view. The view concept helps to focus on a single property of a component and thus helps to handle complexity. In this paper, we focus only on the functional views and on the failure views already explained above, which are the results of fault tree analysis of the component. This analysis, the resulting failure specification and failure realization, as well as the relationship between both views will be discussed in the remainder of this paper. 

For fault tree analysis, the starting point is to specify an undesirable serious situation, called the top effect, and then to consider all possible causes that could produce it. For example, the specified top effect could be the over-pressurization of a chemical reactor. Possible causes could include a reduction or loss of coolant, excess catalyst, an ineffective pressure control loop, etc. Each possible cause is analyzed further to determine why it occurred. For example, the pressure control loop problem could be to the result of a sensor or control valve malfunction. Thus, FTA is a top-down approach that generates a tree of causal relations, starting with the specified top event and working backward. Standard logic concepts, such as AND and OR, are used in the logic diagrams. 

The previous section served to recall some basic concepts (fault, error, and failure), but the systematic research of failures and the analysis of their effects on the system is achieved through activities such as preliminary hazard analysis (PHA), failure modes and effects analysis (FMEA), fault trees analysis (FTA), etc. 

Both the integrative model by Smillie Ayoub (1975) and the deviation concept by Kjellen (1984a) connect the general systems theory to the sequencing and energy models of accident causation. They encompass technical, organizational and human components of the system. Various methods of system safety analysis (e.g. fault tree analysis, incidental factor analysis) support the identification of technical and human deviations as well as the analysis of the conditions and consequences of these deviations. From the discussion of near misses and conflicts it became clear that frameworks of accident causation should cover all kinds of incidents, thus becoming frameworks of incidents. 

Parameter and data uncertainty as well as uncertainly on the concept of the model should be considered for all models of a PSA, whereas uncertainly on the mathematical and numerical model should be taken into account mainly for the more complex thermal-hydraulics and integral codes. But so far, the uncertainty analysis in a PSA is generally restricted to the consideration of parameter and data uncertainties in the fault-tree and event-tree models. More specifically, the analysis is focused on uncertainties... 

Fault Trees are basically logic trees that show the associations between events. Figure 3.4 shows the Fault Tree concept, andFigure 3.5 shows the analysis symbology. 

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